1. Field of the Invention
This invention relates to a curable resin composition which is capable of yielding a cured film which is resistant to acid and exhibits a good appearance.
2. Description of the Prior Art
As studies on the influence of acid rain have been prosecuted, it has been found that acid rain not only presents an ecological problem, but also adversely influences coatings, such as for motor vehicles, for which good appearance is required. More particularly, acrylic resins have been usually employed as coatings of motor vehicles wherein melamine resins are used as a curing agent for the acrylic resins. It has been found that the melamine resins are very susceptible to hydrolysis with acid rain.
For the past several years, novel types of coating materials have been developed and applied as a topcoating for motor vehicles in place of acrylic-melamine resin compositions in order to cope with the problem caused by acid rain. For instance, Japanese Laid-open Patent Application No. 1-139653 proposes a curing coating composition which comprises a carboxyl group-containing acrylic resin and an epoxy resin or which comprises a carboxyl group-containing acrylic resin and an epoxy group-containing acrylic resin. Moreover, there have been proposed in Japanese Laid-open Patent Application Nos. 62-187749 and 61-60748 curing coating compositions comprising carboxyl group-containing resins or amino group-containing resins and .gamma.-glycidopropyltrialkoxysilanes.
As is known in the art, the mechanism of curing between carboxyl and epoxy groups has been extensively studied. In fact, this curing system ensures a good acid resistance and has been recently put into practice in place of melamine curing systems. The curing reactions between the carboxyl group-containing acrylic resins and .gamma.-glycidopropyltalkoxysilanes include further reactions with siloxanes in addition to the above-mentioned reaction between the carboxyl and epoxy groups.
In general, the surfaces wetted with acid rain become more acidic during the course of drying. The cured coating products obtained through the above curing reaction mechanisms have not been accepted as having a satisfactory resistance to acid rain.